MELAS (Fig. 33) is a mitochondrial cytopathy that occurs secondary to a single point mutation of the mtDNA (most frequently tRNA Leu(UUR) 3243A>G) observed in approximately 80% of cases. However, many other phenotypes are reported in association with this mutation, and the same phenotype noted in MELAS has been reported in association with other mutations.
In patient with MELAS, stroke–like episode is often associated with lesions of T2 high signal in the posterior lobes (arrows:FLAIR).
MELAS shows maternal inheritance in some families. However, mothers who carry the mutation and transmit it to their offspring often remain asymptomatic probably related to heteroplasmy. Histopathological findings of muscle tissue include RRF, fibers with low COX activity and sometimes strongly SDH-reactive blood vessels (Fig. 34).
In the muscle of MELAS, blood vessels may have increased activity of succinic dehydrogenase (SDH) (a: arrow). On electron microscopy, in addition to the Paracrystalline inclusions of mitochondria (b), pericytes of the blood vessels may be packed with mitochondria (c).